[HP50G] DESOLVE Error: Bad Argument Type

[dns.oliv]

I'm trying to solve a differential equation with DESOLVE, but I get bad argument type error message. Numeric, Approx and Complex CAS flags are disabled (actually I disabled all CAS flags).

Here is what I enter on RPN mode:
1. [ 'd1d1y(t)+5*y(t)=2*COS(t/2)' 'y(0)=6/5' 'd1y(0)=-1/2'] + ENTER
2. 'y(t)'
3. DESOLVE (LEFT SHIFT + 4[CALC] + F3[DIFF] + F1[DESOL])
Note: I'm using soft menus (flag 117 set).

Does anybody have any clue why I'm getting the error?